1. Field of the Invention
The invention relates to guide wires and catheters commonly used in human arteries and specifically to improvements thereto incorporating shape-memory alloys.
2. Introduction to the Invention
Guide wires commonly used in human arteries (and particularly in coronary arteries) are commonly fabricated from type 304 stainless steel having a yield strength of about 300,000 psi and an elastic strain limit of about 1.375 percent. Such wires are easily used in relatively straight arteries, but the user experiences difficulties when such wires are used in more torturous arteries. Such wires are particularly difficult to use in torturous distal arteries in which the wire diameter would ideally be less than 0.018 inch. Such wires and catheters are usually, but not necessarily, introduced through the use of a guide catheter and are often slidably and rotationally mounted within a small lumen. This lumen is frequently fabricated from helically wound wire or a polymeric material.
As these devices are inserted into torturous arteries, the core wire is bent and forced against the wall of the inner lumen. Alternatively, if no inner lumen is used then the wire is bent and forced against the artery. In wires smaller than 0.018 inch in diameter, the yield strength of the wire would often be approached and could even be exceeded, resulting in plastic deformation (kinking) of the wire. Even when plastic deformation does not occur, considerable forces are exerted between the wire and the inner lumen or the artery. In this case it is necessary to overcome the static frictional forces to move the wire either slidably (to overcome sliding friction) or rotationally (to overcome rolling friction).
In the process of overcoming these frictional forces, considerable energy is stored in the wire prior to overcoming the frictional forces. Once the frictional forces are overcome, the energy is quickly released resulting in a "jerking" motion and the ensuing lack of control over the wire.
Although numerous coatings or coverings have been used in guide wire applications to lower the frictional coefficients with some degree of success, high forces still exist which tend to damage these coatings and cause plastic deformation of the wire. Stainless steel wires (with higher elastic limits) which have been highly work-hardened exert large stresses upon bending. Using such wires often presents difficulties when trying to maneuver them into branch arteries due to the amount of force (and resulting high stress) required to force a uniform radius into a branch. It would therefore be highly desirable to have a guide wire with the following properties: high-elastic deformation capability, low rolling and sliding frictional resistance, a small radius at low stress on bends, and tactile response.
In the past, shape-memory alloys have been used in medical applications due to the unique physical properties of the alloys U.S. Pat. No. 4,665,906, the disclosure of which is incorporated herein by reference, discloses the fact that materials possessing shape-memory are well-known and can be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. These articles are said to have shape-memory for the reason that upon the application of heat alone, they can be caused to revert or attempt to revert from the heat-unstable configuration to the original, heat-stable configuration, i.e., because of the material, the article "remembers" its original shape.
The alloy possesses shape-memory because the alloy has undergone a reversible transformation from an austenitic metallurgical state to a martensitic metallurgical state upon changes in temperature. An article made from such an alloy is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is transformed from the austenitic state to the martensitic state. The temperature at which this transformation begins is usually referred to as M.sub.s, and the temperature at which it finishes is usually referred to as M.sub.f. When an article thus deformed is warmed to a temperature at which the alloy starts to revert to austenite, referred to as A.sub.s (A.sub.f being the temperature at which the reversion is complete), the deformed article will begin to return to its original configuration.
Many shape-memory alloys are known to display stress-induced martensite when stressed at a temperature above M.sub.s (so that the austenitic state is initially stable), but below M.sub.d (the maximum temperature at which the martensite formation can occur even under stress) wherein an article made from the alloy first deforms elastically and then, at a critical stress, begins to transform by the formation of stress-induced martensite. If the temperature is below A.sub.s, the stress-induced martensite is stable, but if the temperature is above A.sub.s, the martensite is unstable and transforms back to austenite with the article returning (or attempting to return) to its original shape. The extent of the temperature range over which stress-induced martensite is seen, and the stress and strain ranges for the effect vary greatly with the alloy.
Many medical devices using shape-memory alloy rely upon the fact that when the shape-memory alloy element is cooled to its martensitic state and is subsequently deformed, it will retain its new shape, but when warmed to its austenitic state, the original shape will be recovered. U.S. Pat. No. 4,665,906 introduces medical devices using shape-memory alloy which display stress-induced martensite rather than heat-recovery to perform a task, i.e., to do work in a medical device. In these devices the shape-memory alloy component exhibiting stress-induced martensite is deformed into a deformed shape different from a final shape and is restrained by a separate restraining means, removal of the restraining means allowing the component and therefore the device to perform some operation. The disclosure of this patent is therefore limited to the concept of restraining the stored energy within the component of shape-memory alloy--it discloses, in essence, a spring. The disclosure is not suggestive of a medical device capable of high-elastic deformation, exhibiting low rolling and sliding frictional resistance, and which provides a tactile response.
U.S. Pat. No. 4,776,844 discloses a medical tube having an elastic member embedded in the peripheral wall of the tube for keeping the tube straight wherein the elastic member is formed of a high-elastic alloy (a shape-memory alloy). The transformation temperature at which the alloy transforms in phase from the martensite structure to the austenite structure is set at a temperature lower than the temperature at which the medical tube is used. The "high-elastic alloy" is a shape-memory alloy believed to be displaying stress-induced martensite wherein one or more elastic members are embedded in the device--again acting like a spring to prevent the device from buckling. The embedded shape-memory alloy components, as discussed with reference to FIGS. 10-12 of the patent, may also display heat-recovery when the driving member 42 is heated by warm water to again perform work. None of the embodiments suggests a guide wire of shape-memory alloy alone wherein the guide wire is capable of high-elastic deformation and has low frictional resistance which allows tactile control of such a wire when passed alone through an artery or when slidably mounted within a small diameter lumen of a catheter.
A recent (but believed to be unpublished) paper entitled "Medical Applications and Fabrication Processes Using NiTi Alloys" written by Stice, Sievert, Jr., Lind, Andreasen and Vennes discloses many uses of shape-memory alloys. In the section entitled "Orthopedics-Arthroscopic Instrumentation" a curved cannula with an initially straight shape-memory alloy needle is disclosed. The shape-memory alloy component bends due to the property of nickel-titanium called "pseudo-elasticity" which is a term commonly used instead of "stress-induced martensite". This disclosure is the opposite of that described in U.S. Pat. No. 4,665,906 wherein the alloy component was bent and was restrained to a straight position by another member. The disclosure of the paper does not suggest the guiding of the device with a guide wire capable of high-elastic deformation and low frictional resistance necessary for tactile response.
The paper discusses the substitution of shape-memory alloy for stainless steel in a guide wire to at least partially eliminate the use of a helically wound coil recovery member which supports a stainless steel guide wire. Trauma is thought to be caused by the coils. Regardless of whether or not the paper can be considered to be prior art, it does not disclose or suggest the application of shape-memory alloy in small diameter wires, i.e., less than 0.018 inch, where full coil wires are not conventionally used but where the stiff core of wire itself pressing against the wall of an artery at a bend will cause trauma. This trauma is more severe in the case where a full coil envelopes the stainless steel core in large diameter cores but is still caused fundamentally by the stiffness of the stainless steel core wire in small diameter cores not suggested by the paper. It would therefore be desirable to have a guide wire of small diameter which is capable of being bent at low stress levels with high-elasticity such that when inserted into tortuous arterial passages, the forces exerted on the artery walls are low, thereby minimizing trauma to the artery.
The brochure entitled "Radifocus.TM. Guide Wire M" by Terumo disclosed the use of a super-elastic "core material" imbedded in a hydrophilic polymer. In this application the wire forms the core of a composite construction with a thick polymer outer layer which extends beyond the distal end of the core wire. The wire has no provision for a platinum floppy tip. The polymer and the lack of a floppy tip eliminate the possibility of shaping the tip to enhance steerability. The absence of a heavy metal at the distal end also makes it difficult or impossible to see in a human artery under fluoroscopy; steering the wire to a particular branch or lesion is not possible since the wire cannot be seen. The relatively large diameter Terumo wire is therefore similar in structure and function to the device described earlier in U.S. Pat. No. 4,776,844 where the shape-memory alloy is embedded in the device and acts like a spring to prevent the device from buckling.